<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果=<span></span></div>
<script>
  function f(){
    let i = document.querySelector("#i1");
    let s = document.querySelector("span");
    if (isNaN(i.value)){
      s.innerText="憨批!输入错误!"
      return;
    }
    /*s.innerText = i.value + i.value;*/
    s.innerText = parseFloat(i.value) + parseFloat(i.value) + "";
  }
</script>
</body>
</html>